We previously have shown that muscarinic acetylcoline receptor activation causes a long-lasting loss of muscarinic binding sites in three different cell systems. In our present study, we plan to investigate the mechanisms of neuroreceptor activation and regulation in the neuroblastoma clone NIE-115. Our experiments focus on how receptor activation decreases the steady-state number of binding sites per cell as much as 90%. Using the specific binding of the muscarinic antagonist 3H-quinuclidinyl benzilate, we plan to measure changes in receptor concentrations in response to regorously controlled independent variables. Our experiments are designed to answer the following fundamental questions: (1) What is the quantitative influence of receptor activation on the rates of receptor appearance into the surface membrane and of receptor disappearance from the surface membrane? (2) Is receptor binding site loss due to a reversible "masking" of receptors or is it due to loss of receptor molecules? (3) What is the role of metabolic processes and subcellular assemblies in regulation of neuroreceptors?